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ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001), Brittle, Metallic
Al 2SiO 5Michael OâDonoghue , Gems, Sixth edition (2006)
BeAlSiO 4(OH)Michael OâDonoghue , Gems, Sixth Edition (2006) More from other references